Hey Firesong...
I completely understand where you're at and what you're going through. I went through something really, really similar. And I had, and am having (although it's getting easier), a hard time getting past it. It's been 5 months over here (although we're on week 3 of no contact)
Here's what I've learned; you can take it or leave it, but know that it's meant in the most loving of ways.
1. It doesn't help to think that he never loved you. Bottom line? He's not capable of loving anything other than drugs and partying right now. Do yourself a favor and read the sticky 'What addicts do.'
2. I recently had a similar situation with a picture that popped up on social media. My AXBF looked happy as well. And I had the same thought. Bottom line? You're not gonna know. And truthfully, kid gloves off, it doesn't matter. Because unless 'someone who's not solid in their recovery' is on your list of qualities you look for in a person, he's not the person you knew when he was clean. (But, from me to you, in all probability, and ESPECIALLY with heroin, he's not clean. Chemically, neurologically, it doesn't work that way. And even if he's not back on dope and is using other substances "so he doesn't use dope", then he's a ticking time bomb.)
3. Does he hate you? Also no. Likely not. But remember, you are a living, breathing reminder (who is fully capable of offering opinions contrary to the ones he probably wants to hear) of the person that he knows he can be. But is choosing not to be. And even if you're in the camp that believes that addiction is a disease and he has no control, then he is choosing not to manage his disease. That's his choice. I am learning (for my own life) that our current realities are a result of the choices that we've made. Same. Goes. For. Him.
There's also probably a lot lot LOT of shame going on for him.
I have found that dealing with a breakup of any kind with an addict is doubly hard. It's not like the person is just a jerk. You're dealing with someone YOU NEVER KNEW--except they look a lot like the person you did know. It's like being in the twilight zone. And then you start thinking you're crazy (been there).
You're not.
But protect yourself from seeing things like that: block him on social media. Or if you can't bring yourself to do that(which I fully understand), hide him. And his friends. (Yes, it makes your timeline lonely...but you know what? Better than feeling this way)
When I first started this process, everyone said "Make this about YOU" and I was like, "wth does that even MEAN? Can't you all see this beautiful man is self destructing in front of your eyeballs?" I couldn't keep food down for four weeks. I lost 20 lbs. I had to zzzQuil myself to sleep for longer than "recommended." I got a dog (in part to replace his dog who lived here with him--and me), but also so I would have someone to get up and out for. It was so so SO hard.
And then slowly (and with the help of everyone here, and Naranon, and a therapist), I began to realize how dysfunctional his behavior was. And that even though I cared deeply, the actual reason behind his poor behavior didn't matter. I don't want someone who says mean things to me. I don't want someone who does the things he did to me. And that's leaving behind the fact that I don't want someone who uses.
Maybe for today(don't worry about longer than that), try to stop expecting him to behave like the clean, healthy version that you knew. Sadly, that's not the world you live in now. (I know how hard that is. I had to do it too) But, when you stop expecting him to behave that way, you also have the added benefit of not being disappointed, hurt, and angry when he doesn't.
You're welcome to PM me too, if you want.
(Biiiiiiig hug)